W. Doring

Response of a PbWO/sub 4/-scintillator array to electrons in the energy regime below 1 GeV

PbWO/sub 4/ has been selected as the most appropriate scintillator material for future calorimeter projects at LHC to detect high energy photons in the multiple GeV range. In spite of the low yield of scintillation light, the applicability of the scintillator to detect photons and particles at energies well below 1 GeV has been studied using monoenergetic electrons between 180 and 855 MeV. The individual crystals (20/spl times/20/spl times/150 mm/sup 3/) are characterized by the optical quality and the response to low energy /spl gamma/-sources using hybrid photo diode tubes. The energy resolution of the EM-shower deposited by electrons in a 3/spl times/3 crystal matrix amounts to /spl sigma//E=2.39%//spl radic/E+0.20% (E given in GeV). The obtained time resolution of /spl sigma//spl les/130 ps allows photon/particle discrimination via time-of-flight measurement.

Detection of monochromatic photons between 50 and 790 MeV with a PbWO/sub 4/-scintillator array

The performance of PbWO/sub 4/ as a fast and compact scintillator material for calorimetry at energies well below 1 GeV has been investigated using a collimated beam of monoenergetic photons with an energy between 50 and 790 MeV provided by the tagged photon facility of MAMI at Mainz. In continuation of previous experiments with electrons, a larger matrix of 5/spl times/5 PbWO/sub 4/-crystals (20/spl times/20/spl times/150-200 mm/sup 3/) has been used to reconstruct the electromagnetic shower. The scintillators were selected according to their optical quality and response to low energy /spl gamma/-sources. The array was operated at a stabilized temperature of T=8/spl deg/C and read-out individually by photomultiplier tubes. The achieved excellent energy resolution amounts to /spl sigma//E=1.54%//spl radic/E+0.30% (E given in GeV) and has proven the applicability of PbWO/sub 4/ as a high quality scintillator material even at medium energies. The experimental data have been reproduced by GEANT3 simulations.

Radiation Hardness and Recovery Processes of PWO Crystals at

For the first time, full size lead tungstate crystals of different suppliers, quality and dopant concentration have been irradiated with gamma-rays at low temperatures down to -25degC at IHEP Protvino. In contrast to the behavior at room temperature, increased damage and extremely slow recovery processes have been observed. These first results are discussed in the light of several very different interpretations. Further more sensitive measurements are under preparation. The outcome will have a strong impact on the presently assembled ALICE-PHOS detector and the design of the EM calorimeter of PANDA at the future FAIR facility.

-25\, ^{\circ}

The study of the performance and application of PbWO/sub 4/ in electromagnetic calorimetry at energies far below 1 GeV has been continued. The significantly improved optical and scintillation properties of 15 cm long Nb/La-doped crystals, optimized for the ECAL/CMS calorimeter, are documented. The lineshape, energy and time response of a 5/spl times/5 matrix are tested with monoenergetic photons up to 790 MeV energy and compared to previous measurements. First attempts have been made to enhance the scintillation yield by suitable dopants (Mo,Tb) for applications at very low photon energies. As a first large scale project at medium energies, the proposed concept for a compact photon spectrometer to be implemented into the ANKE magnetic spectrometer at COSY (KFA Julich) is illustrated.

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For the first time, the response function to high-energy photons of a 3 times 3 matrix comprising large volume LYSO crystals was measured using energy marked photons provided by the tagged photon facility of MAMI. The crystal quality was determined based on the optical transparency, the intrinsic radioactivity and the luminescence yield. Energy and time resolutions for photons up to 490 MeV photon energy have been deduced from the reconstruction of the electromagnetic shower deposited into the crystal array and the data delivers very promising results.

Electromagnetic calorimetry with PbWO/sub 4/ in the energy regime below 1 GeV

The light output of PbWO/sub 4/ crystals, produced at BTCP, has been significantly enhanced by an improved manufacturing technology. First large size crystals of 150 and 200 mm length have been tested assembled in 3/spl times/3 arrays using tagged photons between 64 MeV and 520 MeV energy. The scintillation light has been detected either with commercial photomultiplier tubes or new developed large area silicon avalanche photo diodes of 10/spl times/10 mm/sup 2/ active area. Excellent energy resolutions have been obtained in both cases. For the first time, a timing resolution well below 1 ns has been achieved with diode readout.

High-Energy Photon Detection With LYSO Crystals

The response function of a 3 times 3 matrix of 200-mm-long PWO-II crystals has been measured with energy-marked photons up to 675 MeV energy. For the first time the crystals were readout individually with one large area avalanche photo diode of the second generation operating at a temperature of 0degC. The reconstructed electromagnetic shower distributions show excellent energy resolutions down to 40 MeV. Reducing the temperature and increasing the light collection by an additional large-area avalanche photo diode can achieve further improvement.

High resolution calorimetry with PWO-II

A new charged-particle veto detector for the photon spectrometer TAPS has been designed, tested and manufactured. The hexagonal plastic scintillators of 5 mm thickness are read-out via green emitting wavelength-shifting fibers and multi-anode photomultipliers. The concept, the performance of prototypes tested with high energy electrons and cosmic muons and the final design are presented. The limitations of fast and compact multi-anode metal case photomultipliers are discussed in detail.

Performance of PWO-II Prototype Arrays for the EMC of PANDA

The PANDA detector will be one of the major experimental installations at the future acceleration facility FAIR at GSI (Darmstadt) for hadron physics studies using a cooled antiproton beam. A new type of the lead tungstate crystals (PWO-II) was developed as the scintillation material for the electromagnetic calorimeter of the target spectrometer of PANDA, which has to provide photon detection in the energy range from 10MeV up to 10GeV with excellent energy resolution operating at high interaction rates. The new PWO-II crystals of improved quality are characterized by light yield of 17–22 photoelectron per MeV of deposited energy and radiation induced absorption coefficient at 420 nm (luminescence maximum) below 1m−1 (integral dose 3krad), both determined at room temperature. The crystals were optimized with respect to light yield and radiation hardness for operation at low temperatures down to T=−25°C. The paper presents studies of the quality parameters of two pre-production lots of 120 and 600 PWO-II full size crystals comprising samples of 11 different shapes for the barrel part of the calorimeter.

The new charged-particle veto detector for the photon spectrometer TAPS

Detailed studies of the radiation hardness of cooled high-quality PWO-II crystals for the electromagnetic calorimeter of the target spectrometer of PANDA at the future FAIR facility have been performed. Operating at low temperatures, such as T=−25°C, slows down significantly any recovery process in competition with the radiation damage. The investigations show, that one can overcome this problem by extremely radiation hard crystals selected at room temperature. In spite of light losses, the effective gain of the light yield at T=−25°C still remains at a factor close to three.